IAEA-TECDOC-1466
Physics and technology of inertial fusion energy targets, chambers and drivers
Proceedings of a technical meeting held in Daejon, Republic of Korea, 11–13 October 2004
September 2005
IAEA-TECDOC-1466
Physics and technology of inertial fusion energy targets, chambers and drivers
Proceedings of a technical meeting held in Daejon, Republic of Korea, 11–13 October 2004
September 2005
The originating Section of this publication in the IAEA was:
PhysicsSection
International Atomic Energy Agency Wagramer Strasse 5
P.O. Box 100 A-1400 Vienna, Austria
PHYSICS AND TECHNOLOGY OF INERTIAL FUSION ENERGY TARGETS, CHAMBERS AND DRIVERS
IAEA, VIENNA, 2005 IAEA-TECDOC-1466 ISBN 92–0–108405–6
ISSN 1011–4289
© IAEA, 2005
Printed by the IAEA in Austria September 2005
FOREWORD
The third IAEA Technical Meeting on Physics and Technology of Inertial Fusion Energy Targets and Chambers took place 11–13 October 2004 in the Yousung Hotel Daejon, Republic of Korea. The first meeting was held in Madrid, Spain, 7–9 June 2000, and the second one in San Diego, California, 17–19 June 2002.
Nuclear fusion has the promise of becoming an abundant energy source with good environmental compatibility. Excellent progress has been made in controlled nuclear fusion research on both magnetic and inertial approaches for plasma confinement. The IAEA plays a pro-active role to catalyze innovation and enhance worldwide commitment to fusion. This is done by creating awareness of the different concepts of magnetic as well as inertial confinement. The International Fusion Research Council (IFRC) supports the IAEA in the development of strategies to enhance fusion research in Member States. As part of the recommendations, a technical meeting on the physics and technology of inertial fusion energy (IFE) was proposed in one of the council meetings.
The objective of the technical meeting was to contribute to advancing the understanding of targets and chambers for all proposed inertial fusion energy power plant designs. The topics to be covered were: Target design and physics, chamber design and physics, target fabrication injection and Tritium handling, assessment of safety, environment and economy aspect of IFE. It was recognized by the International Advisory Committee that the scope of the meeting should also include fusion drivers. The presentations of the meeting included target and chamber physics and technology for all proposed IFE plant concepts (laser driven, heavy-ion driven, Z-pinches, etc.). The final Research Coordination Meeting of the Coordinated Research Project on Elements of Power Plant Design for Inertial Fusion Energy, including further new results and achievements, followed the technical meeting.
Twenty-nine participants from 12 countries participated in this meeting. Twenty-four presentations on the topics of plant design, drivers, targets and chambers were given. The topic of drivers, including heavy ion drivers, has been added to the outline of the programme.
Five presentations were given on this topic. One of these presentations was accepted and presented as an overview paper during the 20th Fusion Energy Conference in Vilamoura, Portugal.
The International Advisory Committee consisted of S. Nakai, T. Norimatsu, E. Koresheva, W. Meier, J.M. Perlado, R. Khardekar, P. Joyer, and H.J. Kong. Acknowledgements should be directed to the local organizer, H.-J. Kong, who ensured that the technical work was supported by great hospitality.
The IAEA officer responsible for the preparation of this publication was G. Mank of the Division of Physical and Chemical Sciences.
EDITORIAL NOTE
The papers in these proceedings are reproduced as submitted by the authors and have not undergone rigorous editorial review by the IAEA.
The views expressed do not necessarily reflect those of the IAEA, the governments of the nominating Member States or the nominating organizations.
The use of particular designations of countries or territories does not imply any judgement by the publisher, the IAEA, as to the legal status of such countries or territories, of their authorities and institutions or of the delimitation of their boundaries.
The mention of names of specific companies or products (whether or not indicated as registered) does not imply any intention to infringe proprietary rights, nor should it be construed as an endorsement or recommendation on the part of the IAEA.
The authors are responsible for having obtained the necessary permission for the IAEA to reproduce, translate or use material from sources already protected by copyrights.
CONTENTS
SUMMARY ... 1
POWER PLANT DESIGN
Progress on Z-pinch inertial fusion energy ... 5 C.L. Olson, the Z-Pinch IFE team
Evaluation of a concept of power plant for fast ignition heavy ion fusion ... 11 S.A. Medina, M.M. Basko, M.D. Churazov, P. Ivanov, D.G. Koshkarev,
Yu.N. Orlov, A.N. Parshikov, B.Yu. Sharkov, V.M. Suslinc
The Mercury Laser, a gas cooled 10 Hz diode pumped Yb:S-FAP system for
inertial fusion energy... 23 C. Bibeau, A.J. Bayramian, P. Armstrong, R.J. Beach, R. Campbell,
C.A. Ebbers, B.L. Freitas, T. Ladran, J. Menapace, S.A. Payne, N. Peterson, K.I. Schaffers, C. Stolz, S. Telford, J.B. Tassano, E. Utterback
Progress in inertial fusion energy modelling at DENIM... 29 G. Velarde, O. Cabellos, M.J. Caturla, R. Florido, J.M. Gil, P.T. León,
R. Mancini, J. Marian, P. Martel, J.M. Martínez-Val, E. Mínguez, F. Mota, F. Ogando, J.M. Perlado, M Piera, R. Rodríguez,
J.G. Rubiano, M. Salvador, J. Sanz, P. Sauvan, M. Velarde, P. Velarde
DRIVERS FOR IFE
HALNA DPSSL for inertial fusion energy driver ... 39 O. Matsumoto, T. Kanabe, R. Yasuhara, T. Sekine, T. Kurita, M. Miyamoto,
T. Kawashima, H. Furukawa, H. Kan, T. Kanzaki, M. Yamanaka, T. Norimatsu, N. Miyanaga, M. Nakatsuka, Y. Izawa, S. Nakai Intense heavy ion and laser beams interacting with solid,
gaseous and ionized matter ... 45 D.H.H. Hoffmann, A. Blazevic, P. Ni, O. Rosmej, M. Roth, N. Tahir,
A. Tauschwitz, S. Udrea, D. Varentsov, K.Weyrich, Y. Maron High-quality return relativistic electron beam by intense laser pulse
in a low-density foil plasma ... 49 B. Li, S. Ishiguro, M.M. Skoric, T. Sato
Generation of intense fast proton streams with the use of a picosecond high-power
laser interacting with a double-layer foil target ... 57 J. Badziak, H. Hora, J. Krása, L. Láska, P. Parys, K. Rohlena, J. Wolowski
TARGET, INTERACTION
Kelvin-Helmholtz instability in a viscous thin film past a
nanostructure porous lining... 67 N. Rudraiah
Interactions of subnanosecond laser-pulses with low-density plastic foams ... 77 M. Kalal, J. Limpouch, N.N. Demchenko, S.Yu. Gus’kov, A.I. Gromov,
A. Kasperczuk, V.N. Kondrashov, E. Krousky, K. Masek, M. Pfeifer,
P. Pisarczyk, T. Pisarczyk, K. Rohlena, V.B. Rozanov, M. Sinor, J. Ullschmied
TARGET, FABRICATION
FST technologies for IFE targets fabrication, characterization and delivery... 87 E.R. Koresheva, I.V. Aleksandrova, G.D. Baranov, V.I. Chtcherbakov,
A.I. Kupriashin, V.N. Leonov, A.I. Nikitenko, I.E. Osipov, V.V. Petrovskiy, I.A. Rezgol, A.I. Safronov, T.P. Timasheva, I.D. Timofeev,
S.M. Tolokonnikov, A.A. Tonshin, L.S. Yaguzinskiy
CHAMBER
Possible approaches to rapid control of IFE targets... 97 A.I. Nikitenko, I.V. Aleksandrova, S.V. Bazdenkov, A.A. Belolipetskiy,
V.I. Chtcherbakov, E.R. Koresheva, I.E. Osipov
Investigation of SiC and PbMg target characteristics by the laser
mass-spectrometer... 103 R. Khaydarov, U. Kunishev, E. Tojihanov, M. Kholmyratov, G. Berdiyorov
IFE chamber wall ablations with high-flux pulsed beams including ions
and UV laser lights... 111 K. Kasuya, T. Norimatsu, S. Nakai, A. Prokopiuk, W. Mroz
Chamber clearing study relevance to Z-pinch power plants ... 119 P. Calderoni, A. Ying, M. Abdou
List of Participants ... 125
SUMMARY
It is expected that new megajoule laser facilities which are under construction in the US and France will demonstrate fusion ignition and burn, and, around 2010~2015, gain of energy.
This will be an epoch-making achievement in the history of fusion energy development, which will give us the real means to solve the future energy and environmental problems of the world.
Now the strategic approach toward the final goal, namely fusion energy production on a commercial basis, is required. An inertial fusion energy (IFE) power plant and its development are based on a large number of advanced concepts and technologies, such as drivers, target fabrication and injection, reaction chamber, and remaining system.
The separability of IFE power plant systems means that these concepts and technologies can be developed somewhat independently and later assembled to form a system. Therefore, worldwide efforts in IFE research and engineering development could be organized to be very effective, provided that sharing of information is timely and complete. The contribution and auspices of IAEA in organizing the IAEA-TM (before TCM: Technical Committee Meeting) on IFE-related topics have worked effectively in gathering the scientific and technical informations on IFE and in coordinating the worldwide collaboration.
The outline of the issues which have been discussed at the Third IAEA-TM on Physics and Technology of Initial Fusion Energy Target, Chamber, and Driver are summarized as follows:
1. The progress of the Z-pinch IFE and a new concept of power plant for fast ignition heavy ion fusion have been reported and discussed with much attention. The efforts at the Instituto de Fusión Nuclear - Universidad Politécnica de Madrid (DENIM) on the IFE modeling include safety and environmental issues of IFE together with the target and chamber dynamics. It presents new challenges toward the real evaluation of IFE power plant.
2. The Diode Pumped Solid State Laser (DPSSL) is becoming the most feasible candidate as IFE driver for a power plant. Two most advanced programmes, Mercury at Lawrence Livermore National Laboratory (LLNL) and high average power laser for nuclear fusion application (HALNA) at the Institute of Laser Engineering (ILE) at Osaka University, have been reported and discussed. Multi-beam combining with stimulated Brillouin scattering (SBS) phase conjugation is an essential technique to form a large laser IFE driver with well-configured elemental modules. Excellent results at the Korea Advanced Institute of Science & Technology (KAIST) on this technique have been reported and showed the possibility of practical use of this technology.
3. Interaction physics is still an important issue for improving the target design aiming at more efficient and robust implosion with higher gain, and also for developing an advanced concept such as fast ignition. Many interesting and meaningful results came from GSI- Darmstadt on Intense Heavy Ion and Laser Beams interaction and the Vinca Institute of Nuclear Science in Belgrade. The Institute of Plasma Physics and Laser Micro Fusion in Warsaw and the Czech Technical University in Prague, both of which are investigating the fundamental process of the new field on high energy density physics, have reported on the interaction of intense short pulse laser with plasmas. The hydrodynamic instability problem is an essential issue in designing the efficient implosion of fusion pellet.
Theoretical investigation and an advanced concept to stabilize the surface instability have been presented by the National Research Institute for Applied Mathematics (NRIAM) in Bangalore, India.
4. Target fabrication and injection have been discussed mainly focusing on the technical aspect of the repetitive operation, which is essential for a power plant.
5. Chamber wall response against high flux pulsed energy, and chamber dynamics and clearing study are key issues in designing the IFE reaction chamber. Material studies on SiC and PbMg by using a laser mass-spectrometer were reported from Scientific-Research of Applied Physics at the National University of Uzbekistan. Intensive research works on chamber wall ablation and on chamber clearing were reported from the Technical Institute of Tokyo and UCLA respectively.
Finally it can be concluded that the IAEA-TM on specific topics has been very effective for the promotion of the worldwide activity on IFE-related research and development because of the openness of the discussion and the timeliness of the issues in the process of fusion energy development.
2
POWER PLANT DESIGN
Progress on Z-pinch inertial fusion energy*
C.L. Olson, The Z-Pinch IFE Team**
Sandia National Laboratories, Albuquerque, New Mexico, United States of America
Abstract. The long-range goal of the Z-Pinch IFE programme is to produce an economically attractive power plant using high-yield z-pinch-driven targets (∼3 GJ) with low rep-rate per chamber (∼0.1 Hz). The present mainline choice for a Z-Pinch IFE power plant uses an LTD (Linear Transformer Driver) repetitive pulsed power driver, a Recyclable Transmission Line (RTL), a dynamic hohlraum z-pinch-driven target, and a thick-liquid wall chamber. The RTL connects the pulsed power driver directly to the z-pinch-driven target, and is made from frozen coolant or a material that is easily separable from the coolant (such as carbon steel). The RTL is destroyed by the fusion explosion, but the RTL materials are recycled, and a new RTL is inserted on each shot.
The RTL concept eliminates the problems of a final optic, high-speed target injection, and pointing and tracking N beams (N∼100). Instead, the RTL concept must be shown to be feasible and economically attractive. Results of z-pinch IFE studies over the last three years are discussed, including RTL experiments at the 10 MA level on Saturn, RTL structural studies, RTL manufacturing/cost studies, RTL activation analysis, power plant studies, high-yield IFE target studies, etc. Recent funding by a U.S. Congressional initiative of $4M for FY04 is supporting research on (1) RTLs, (2) repetitive pulsed power drivers, (3) shock mitigation [because of the high- yield targets], (4) planning for a proof-of-principle full RTL cycle demonstration [with a 1 MA, 1 MV, 100 ns, 0.1 Hz driver], (5) IFE target studies for multi-GJ yield targets, and (6) z-pinch IFE power plant engineering and technology development.
1. Introduction
Z-Pinch Inertial Fusion Energy (IFE) extends the single-shot z-pinch inertial confinement fusion results on Z to a repetitive basis with high-yield fusion targets for the purpose of producing an attractive and economical power plant [1]. Z-Pinch IFE is relatively new, and has become part of the IFE community over the last five years. Z-Pinch IFE has been part of the 1999 Snowmass Fusion Summer Study, the IAEA Cooperative Research Project on IFE Power Plants, the 2002 Snowmass Fusion Summer Study, the FESAC 35-year Plan Panel (2003) [2], and the FESAC IFE Panel (2004) [3].
Z-Pinch IFE has an attractive potential based on what has already been accomplished with pulsed power driven z-pinches:
_______________
* Sandia is a multiprogramme laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Dept. of Energy under contract No. DE-AC04-94AL85000.
** G. Rochau, S. Slutz, C. Morrow, R. Olson, A. Parker, M. Cuneo, D. Hanson,G. Bennett, T. Sanford, J. Bailey, W. Stygar, R. Vesey,T. Mehlhorn,K. Struve,M. Mazarakis, M. Savage, A. Owen, T. Pointon, M. Kiefer, S. Rosenthal, L. Schneider, S. Glover, K. Reed, G. Benevides, D. Schroen, W. Krych, C. Farnum, M. Modesto, D. Oscar, L. Chhabildas, J. Boyes, V. Vigil, R. Keith, M. Turgeon, B. Smith, B. Cipiti, E. Lindgren, D. Smith, K. Peterson, V. Dandini, D. McDaniel, J. Quintenz, M. Matzen, J.P. VanDevender, W. Gauster, L. Shephard, M. Walck, T. Renk, T. Tanaka, M. Ulrickson, P. Peterson, J. De Groot, N. Jensen, R. Peterson, G. Pollock, P. Ottinger, J. Schumer, D. Kammer, I. Golovkin, G. Kulcinski, L. El-Guebaly,G. Moses, E. Mogahed, I. Sviatoslavsky, M. Sawan,M. Anderson, R. Gallix, N. Alexander, W. Rickman, H. Tran, P. Panchuk, W. Meier, J. Latkowski, R. Moir, R. Schmitt, R. Abbot, M. Abdou, A. Ying, P. Calderoni, N. Morley, S. Abdel-Khalik, D. Welch, D. Rose, W. Szaroletta, R. Curry, K. McDonald, D. Louie, S. Dean, A. Kim, S. Nedoseev, E. Grabovsky, A. Kingsep, V. Smirnov.
• 1.8 MJ of x-rays are routinely produced on Z
• pulsed power has low demonstrated cost (∼$30/J)
• pulsed power drivers have high efficiency –15% for wall plug to x-rays on Z, and this can be optimized to an even higher value
• capsule compression ratios of 14–21 have been achieved with the double-pinch target, and up to 8 × 010 DD neutrons have been demonstrated with the dynamic hohlraum target — this is almost an order of magnitude larger than that demonstrated with any other indirect- drive target with any other driver
• repetitive pulsed power using RHEPP magnetic switching technology has been demonstrated at 2.5 kJ @ 120 Hz (300 kW average power)
In the following, the Z-Pinch IFE system is described, the Recyclable Transmission Line (RTL) concept is discussed, and the status of current research is presented.
2. Z-Pinch IFE concept
The goal of Z-Pinch Inertial Fusion Energy (IFE) is to produce an economically attractive power plant using a repetitive pulsed power driver and z-pinch-driven fusion targets. While many schemes have been proposed [1], the most enduring appears to use the Recyclable Transmission Line (RTL) concept [4], in which an RTL connects the repetitive driver directly to the target, and the fusion explosions are contained in a thick-liquid wall chamber. A unique feature is that high-yield fusion targets (∼3GJ) would be used at the low rep-rate of 0.1 Hz per chamber. The matrix of choices available for a Z-Pinch IFE power plant is summarized in Figure 1.
Z-Pinch Driver: _ Marx generator/ magnetic switching linear transformer driver water line technology (RHEPP technology) (LTD technology)
RTL (Recyclable Transmission Line): _ frozen coolant immiscible material
(e.g., Flibe/electrical coating) (e.g., carbon steel)
Target: _ double-pinch dynamic hohlraum fast ignition
Chamber: _ dry-wall wetted-wall thick-liquid wall solid/voids (e.g., foam Flibe)
Figure 1. Z-Pinch IFE Power Plant – matrix of choices (dashed line shows preferred choices).
Note that several options are available for each part of the power plant – the driver, the RTL, the target, and the chamber type:
• Three possible repetitive driver technologies include Marx generator/water line technology, RHEPP magnetic switching technology, and Linear Transformer Driver
6
(LTD) voltage-adder technology. The LTD approach is compact, requires no oil or water storage tanks, and is the present preferred choice for z-pinch IFE.
• The RTL can be made of frozen coolant (e.g., Flibe) or a material that is immiscible in the coolant (e.g., carbon steel). The latter is the present preferred choice for z-pinch IFE.
• Of several z-pinch target options (double-pinch, dynamic hohlraum, fast ignition, etc.), the dynamic hohlraum target is the present preferred choice for z-pinch IFE.
• The chamber may be dry-wall, wetted-wall, thick-liquid wall, or solid/voids (e.g., foam Flibe). The thick-liquid wall chamber is the preferred chamber for z-pinch IFE.
3. Recyclable Transmission Line (RTL)
The RTL concept, as shown schematically in Figure 2, is to make a low-mass RTL that connects the pulsed power driver to the z-pinch-driven target. The RTL would enter the chamber through a single hole at the top of the chamber (∼1 meter radius), and extend into the chamber a distance of two or more meters. Note that the RTL would bend at the top of the chamber, and upper shielding would be placed above it. The RTL concept alleviates the usual problems of a final optic, pointing and track N beams (N∼100), and high-speed target injection. In contrast to those problems, the RTL concept must be shown to be viable and economically attractive. Issues associated with the RTL include movement (required accelerations are low); RTL electrical current initiation; RTL low-mass limit and electrical conductivity; structural properties; mass handling; shrapnel; vacuum/electrical connections;
activation; waste stream analysis; shock disruption to liquid walls; manufacturing/cost;
optimum configuration (inductance, shape, etc.); power flow limits for magnetic insulation;
effects of post-shot EMP, debris, and shrapnel up the RTL; and shielding of sensitive accelerator parts. Initial experiments at the 10 MA level on Saturn have been successfully used to study the electrical current initiation in the RTL, the RTL low-mass limit, and the RTL electrical conductivity [4].
4. Z-Pinch IFE development areas
The development of Z-Pinch IFE has been organized into six research areas as follows:
1. RTL: The present approach is to use a carbon steel RTL, with a total mass of about 50 kg, in a chamber with a pressure of 10–20 Torr. The structural properties of the RTL set the pressure limit. Power flow properties of the RTL are being investigated. The major concern involves electrode heating, the formation of surface plasmas, accurate determination of the electrical conductivity, magnetic field diffusion into the electrode material, and motion of the electrode material during the power pulse. 2D and 3D codes are being used to evaluate these effects.
2. Repetitive pulsed power drivers: Although other potential repetitive pulsed power technologies are being assessed, the present approach is to use a Linear Transformer Driver (LTD) voltage adder accelerator. In the LTD concept, a series of compact low-inductance capacitors are charged directly in parallel, in cylindrical formation. A series of switches next to the capacitors, and in the same cylindrical formation, switch the charged capacitors to directly apply voltage to a single, inductively-isolated gap. Several such cells are combined in a voltage-adder formation to reach high voltage. Progress is being made on an LTD PoP (Proof-of-Principle) accelerator at 1MA, 1MV, 100 ns, 0.1 Hz. Individual cells at the 100 kV, 1MA, 100 ns level are being designed and constructed.
Figure 2. The RTL (Recyclable Transmission Line) concept.
3. Shock mitigation: The fusion yields envisioned for Z-Pinch IFE are large (∼3GJ) compared to the other IFE approaches that typically use yields ≤0.4 GJ. Therefore, shock mitigation (in the thick liquid walls) to protect the structural chamber walls is an issue that must be addressed. This is being modeled in scaled experiments with a shock tube and water layers, and with explosives and water jets. Code calculations are being performed with the goal of validating the codes with the experiments, and then using the codes to predict effects for a full-scale Z-Pinch IFE power plant.
4. PoP Experiment Planning: The Z-PoP experiment is in the planning stages. It is based on a 1 MA, 1 MV, 100 ns, 0.1 Hz LTD accelerator, as mentioned above, that is under development. Z-PoP would use this driver, together with an RTL and a z-pinch load (∼5 kJ), and would be automated to run at 0.1 Hz. The procedure would be to insert an RTL and a z- pinch load, fire, remove the remnant, reload, and repeat the process. Robotic systems are being investigated to perform these functions for Z-PoP.
5. Targets for Z-Pinch IFE: The dynamic hohlraum target is the preferred target for Z-Pinch IFE, although other targets (double pinch, fast igniter, etc.) are also being considered. Based on Lasnex simulations and analytic scaling studies, fusion yields of 3 GJ may be obtained with this target with gains G of 50–100. This gain, coupled with a driver efficiency (η) that is already 15% (and might be optimized to 25% or more in the future), gives a favorable ηG ∼ 15 or more. This high value of ηG ensures a favorable power plant operating scenario.
6. Z-Pinch power plant technologies: An initial multi-chamber Z-Pinch power plant study named ZP-3 was completed to establish one complete (but non-optimized) 1000 MWe power plant scenario [5]. This concept assumed Marx generator/ water line technology for the
Thick liquid region Thick
liquid region Chamber structural wall
Upper shielding
RTL
z-pinch target Connects to pulsed
power driver
8
pulsed power driver, an RTL to connect the driver to the target, a dynamic hohlraum target, and a thick-liquid wall chamber. If each RTL is ∼50 kg, then a one-day supply would be 5,000 tons. This is the inventory needed for the power plant, and it would be recycled constantly. (For comparison, the one-day waste from a coal-fired power plant is about 5,000 tons.) Activation studies indicate that a one-day supply of RTLs should permit a sufficient cool-down time for the RTL material. Further studies regarding activation, waste stream analysis and chamber clearing are in progress.
4. Conclusions
A Z-Pinch IFE program is underway, and substantial progress is being made in all research areas (RTLs, repetitive pulsed power LTD drivers, shock mitigation, PoP experiment planning, target designs for high yield, and thick-liquid wall power plant design).
REFERENCES
[1] OLSON, C.L., "Z-Pinch Inertial Fusion Energy," in the Landholt-Boernstein Handbook on Energy Technologies (Editor in chief: W. Martienssen), Volume VIII/3, Fusion Technologies (Edited by K. Heinloth), Springer-Verlag (Berlin-Heidelberg) in press (2004). [Includes an extensive list of references.]
[2] US DOE FESAC Report, “A Plan for the Development of Fusion Energy,” DOE/SC- 0074, March 2003.
[3] US DOE FESAC Report, “A Review of the Inertial Fusion Energy Program,” DOE/SC- 0087, March 2004.
[4] SLUTZ, S.A., OLSON, C.L., and PETERSON, P., "Low Mass Recyclable Transmission Line for Z-Pinch Driven Inertial Fusion," Phys. Plasmas 10, 429 (2003).
[5] ROCHAU, G.E., et al., "ZP-3, a Power Plant Utilizing Z-Pinch Fusion Technology,"
IFSA 2001 (Editors: K. Tanaka, D. Meyerhofer, J. Meyer-ter-Vehn), Elsevier, 706 (2002).
Evaluation of a concept of power plant for fast ignition heavy ion fusion
S.A. Medin1, M.M. Basko2, M.D. Churazov2, P. Ivanov1, D.G. Koshkarev2, Yu.N. Orlov3, A.N. Parshikov1, B.Yu. Sharkov2, V.M. Suslinc3
1 Institute for High Energy Densities, Moscow, Russian Federation
2 Institute for Theoretical and Experimental Physics, Moscow, Russian Federation
3 Keldysh Institute for Applied Mathematics, Moscow, Russian Federation
Abstract. The requirements of the heavy ion fusion power plant concept are based on the fast ignition principle for fusion targets. The cylindrical target is irradiated by a hollow beam in a compression phase and subsequently by a powerful ignition beam for initiation of the burning phase. The ignition is provided by the high energy 100 GeV Ptions of different masses and charge states accelerated in RF-linac. The efficiency of the driver is evaluated as 0.25. The main beam delivers 7.1 MJ energy and the ignition beam ~0.4 MJ to the target.
A cylindrical DT filled target provides 750 MJ fusion yield, of which 17 MJ appear in X-rays, 187 MJ in ionized debris and 546 MJ in neutrons. The multiplication blanket coefficient is calculated as 1.117, so the total energy release is estimated by the value of 814 MJ per one shot. The repetition rate is taken as 2 Hz per reactor chamber. The first wall of the reactor chamber employs a thin liquid wall design, particularly the wetted porous design. The lithium-lead eutectic is used as a coolant, with an initial surface temperature of 550oC. Evaporation of liquid film under X-ray irradiation, revaporisation under debris impact and vapor condensation are computed.
The computation of neutronics results in a blanket energy deposition with a maximum density of 20 MJ/m3. Pulsations of stresses in the blanket tubing and of pressure in the coolant are evaluated. The heat conversion system consisting of three coolant loops provides the net efficiency of the power plant of 0.37.
1. INTRODUCTION
Principal aspects of DT-fusion by the use of a heavy ion driver are determined mainly by two design factors: the type of target drive and the blanket structure. At the moment the most advanced concept, HYLIFE-II [1, 2], is based on the indirect drive of the target and the thick liquid wall of the blanket structure. The advantages of this concept are the mitigation of problems of driver–reactor chamber interface and reactor chamber materials.
The difficulties involved are the organization of radiation-proof pockets of liquid jets and the demand of higher gains in target burn. In ref. [3, 4] an alternative concept of heavy ion fusion has been proposed. In this concept the direct drive of a cylindrical target in fast ignition mode and a thin liquid wall design of the blanket are accepted. This approach is characterized by a simple driver–reactor chamber interface and a moderate value of gain demanded. The problems of the FIHIF concept (fast ignition heavy ion fusion) are connected to the rigid physical conditions of fast ignition scenario implementation and the heavy ion driver length.
In this paper new data on the target energy release and the reactor chamber response are given by using developed numerical codes. The output parameters of the FIHIF power plant are corrected.
2. GROUND PLAN AND HIGH-POWER DRIVER
The ground plan of the FIHIF power plant is outlined in Fig. 1. The length of the main linac is of the order of 10 km. The diameter of storage and compression rings is equal to 1 km. The area occupied by the reactor and turbo generator building, as well as by the cooling towers, is of the same order of magnitude.
The driver is composed of the following major parts [4]. Ion sources for four Pt isotopes with plus and minus charge states are arranged in 8 groups of 4 devices each.
Figure 1. Ground plan outline for FIHIF power plan.
In the main linac the ion energy is increased to 100 GeV. After this the ions with different charges and masses are separated into 8 beams, which are compressed in two stages: in storage rings and in exit sections by the time-of-flight method. The final summation of 8 beams in an individual transfer line results in a single bunch of 0.2-ns-duration delivered to the compressed target. The preliminary compression of the target is accomplished by the hollow beam which carries only Pt192+ ions. This beam is temporally profiled over a duration of 75 ns with a maximum current of 1,6 kA. The general parameters of the FIHIF driver are given in Table 1.
Table 1. General parameters of FIHIF driver Ions Pt192+,−,194,196,198 Ion energy , GeV 100
Hollow compressing beam
Energy, MJ 7.1
Duration , ns 75 (profiled) Maximum current, kA 1.6
Rotation frequency,
GHz 1
Rotation radius, mm 1,33-1,89 Focal spot radius, mm 0,65-1,33 Ignition beam
Energy, MJ 0.4
Duration, ns 0.2
Maximum current, kA 20 Spot radius, µm 50 Linac Main linac length, km 10 Repetition rate, Hz 8 Driver efficiency 0.25
Reactor and turbogenerator building
Reactor and turbogenerator building
Transfer lines for ignition beam Transfer lines for ignition
beam
Storage ring Storage
ring Compression
ring Compression
ring
Auxiliary linac Auxiliary
linac Transfer line for compressing beam, PtTransfer line for compressing beam, Pt++192192
Ion sources and low energy linac tree
Ion sources and low energy linac tree
Main linac Main linac 10 km 10 km
Storage rings Storage
rings
+
Pt192-198Pt192-198
Pt192-198Pt192-198 -
12
The repetition rate of the FIHIF driver is taken as 8 Hz, which provides 2 shots per second in each of 4 reactor chambers. The evaluated nominal driver efficiency is equal to 0.25. The operational characteristics of the driver equipment are feasible on the basis of today’s developments in acceleration technologies.
3. CYLINDRICAL TARGET
In the present consideration the target has a simple design. The DT fuel cylinder with a radius of 0.112 cm and a length of 0.71 cm is surrounded by a lead shell with a radius of 0.4 cm and a length of 0.8 cm (see Fig. 2). The length of the target is matched to the stopping range of 100-GeV Pt192+ ions deposited by the compressing hollow beam. The masses of DT fuel and lead are equal to 5.6 mg and 4.44 g, respectively.
Pb shell
R=4mm
Figure 2. Cylindrical target for FIHIF concept.
The parameters of target compression and burn were determined successively in separate simulations [5]. The burn fraction amounts to 0.39, providing a fusion energy of 750 MJ and a gain of 100. This energy is partitioned among 580 MJ in neutrons, 153 MJ in debris and 17 MJ in X-rays.
200 400 600 800 1000 1200
0 10 20 30 40 50
X-ray energy, MJ; Power, TW; Temperature, eV
Time, ns
Figure 3. Temporal profiles of X-ray characteristics for an FIHIF cylindrical target. Dashed line: temperature, solid line: power, dotted line: energy.
X-ray radiation is originally absorbed by target material and then reradiated by lead plasma.
This causes the decrease of X-ray temperature and delay and lengthening of the X-ray pulse.
In Fig. 3 the profile of an X-ray pulse is drawn according to [5]. The duration of the pulse is very long and exceeds 0.7 ms, the mean X-ray temperature is equal to 30 eV.
4. REACTOR CHAMBER DESIGN
The general design of the reactor chamber is shown in Figure 4. The chamber consists of two cylindrical sections: the upper smaller section in which the target explosion takes place and the lower section into which sprayed jets of coolant are injected. The diameters of the sections are 8 m and 16 m respectively. Such a configuration prevents an over-pressurization after the micro-explosion and provides a high rate of vapour condensation on sprayed jets.
Figure 4. Reactor chamber for an FIHIF power plant.
The coolant is eutectic Li17Pb83 with a temperature of 823K. The saturation density of vapour corresponding to this temperature is equal to 1018m–3, according to [6]. Under this condition the Pt ion beam is not deteriorated by an atmosphere in the reactor chamber.
The first wall and the blanket are of conventional design. The liquid film is formed at an SiC porous wall. In the blanket the tubing is made of vanadium alloy. The structural wall is manufactured of HT-9 steel.
5. CHAMBER RESPONSE TO X-RAYS AND ION DEBRIS
We suppose that X-ray radiation is described by Plank distribution, with temperature and power, depending on time, as shown in Fig. 3. The response of a thin liquid protected layer to X-ray radiation is simulated numerically by the Lagrangian 1D hydrodynamic code in cylindrical geometry. This code solves equations of conservation of mass, momentum and energy with viscosity dissipation and heat transfer in the film. The energy deposition of X-ray radiation in the film is computed by means of a mass attenuation coefficient for eutectic Li17Pb83 determined by way of individual substance data, depending on photon energy, given in ref. [7]. The wide-range equation of state is taken from ref. [8] only for lead, because the presence of lithium is neglected in thermodynamic properties. In this equation of state the vaporization and the first ionization of lead is taken into account.
The energy deposition of X-ray radiation occurs immediately in thin liquid layers and later in the vapour which expands from the liquid surface.
14
0.0e0 2.0e2 4.0e2 6.0e2 8.0e2 1.0e3 Time, ns
3.0e3 4.0e3 5.0e3 6.0e3 7.0e3 8.0e3
Temperature, K
Figure 5. Vaporization front temperature versus time.
0 100 200 300 400 500 600 700
0 10 20 30 40 50 60
Vaporization front velocity, m/s
Time, ns
Figure 6. Velocity of vaporization front propagation in the liquid film versus time.
The rate of vaporization depends on X-ray intensity and is strongly influenced by X-ray absorption in this vapour layer. In Figs 5 and 6 the temporal variations of the temperature and the velocity of the vaporization front are presented. After 100ns the vaporization rate begins to decrease despite of the fact that the X-ray intensity continues to rise. This is caused by the intensive absorption of X-ray radiation by vapour and demonstrates a screening effect of the vapour layer. The total mass vaporized by the end of the X-ray pulse is some 1.42 kg (see Fig. 7).
Temperature and density of this vapour are extremely non-uniform. There is a rarefied ionized external zone with a temperature of 3 × 106 K (see Fig. 8) and a density of 10–6 kg/m3. The inner layer, adjacent to the liquid film, has a temperature of 3000 K. The average temperature of vaporized coolant is 1.6 ×105 K, the average density is 0.07 kg/m3.
0 100 200 300 400 500 600 700 0,0
0,2 0,4 0,6 0,8 1,0 1,2 1,4
Vaporized mass, kg
Time, ns
Figure 7. Temporal variation of vaporized mass in the reactor chamber during X-ray exposion.
0.0e0 2.0e-3 4.0e-3 6.0e-3 8.0e-3
Lagrangian coordinates, kg m-2 1.1e3
3.0e3 8.1e3 2.2e4 6.0e4 1.6e5 4.4e5 1.2e6 3.3e6 8.9e6
Temperature, K t = 100 nst = 500 ns
t =1000 ns
Figure 8. The temperature distribution in the vaporized coolant at various times.
The vaporization process is accompanied by shock wave generation in the liquid film. This shock wave travels across the film, with the sound velocity approximately equal to 2000 m/s (see Fig. 9). It crosses the 2 mm wide film in 1 µs and comes to the first wall with an amplitude of 250 MPa. The pressure in the tail of the pressure profile is equal to the saturation pressure at the vaporization front. It can be seen that the saturation pressure decreases in the time following the power variation of X-rays. The displacement of the vaporization front is rather small and not noticeable in Fig. 9 due to the fact that the front velocity (Fig. 6) is much smaller than the velocity of sound.
The pressure at the vaporization front, calculated according to the equation of state, enables us to numerically determine the impulse transmitted to the first wall during the time interval
τ =1000 ns. It turns out to be equal to I = 64 Pa·s. Thus the loading for the first wall is estimated to be P1=I/τ=64 MPa. Taking the first wall to be a thin spherical shell, deformed by this loading, we calculated the corresponding displacement of the porous first SiC wall, which is equal to 4 ×10-6 m. Then we could calculate the maximal pressure on its external surface on contact with coolant. It is equal to P10 = 14 MPa. We took the elasticity limit of the porous SiC wall to be equal to Y0 = 35 MPa. Since the radial stress P10 of this radial stress value turned out to be below the elasticity limit Y0 (the so-called von Mises criterion), we conclude that the first wall remained in the elasticity deformation region under this loading.
The target debris approaches the vapour layer at 10 microseconds.
16
0.0e0 5.0e0 1.0e1 1.5e1 2.0e1 Lagrangian coordinate, kg m-2 0.0e0
1.0e8 2.0e8 3.0e8 4.0e8
Pressure,Pa
t=100 ns t=500 ns t=1000 ns
Figure 9. Shock wave propagation in liquid film.
Further interaction of the debris flow and the vapour layer is determined by the stopping range of 80 keV ions of lead, which is equal to 6 × 10–6 g/cm2 according to the Bethe-Bloch formula of the Coulomb energy losses [9]. Thus the stopping range is much smaller then the vapour layer. Therefore it can be assumed that evaporated coolant is additionally superheated by debris ions. The evaluated resulting temperature of vapor amounts to 230 eV (approximately equal to 2.5 × 106K). The process of additional vaporization and condensation is considered to start at this initial temperature of vapour (see Figs 10, 11).
We describe the evaporation-condensation processes by kinetic relationships [10] in the same way as in ref. [4].
0,0000 0,0005 0,0010 0,0015 0,0020 0,0025 0
2 4 6 8 10
Density, 2.4*1022 cm-3
Time, second
Figure 10. Temporal variation in vapour density in the reactor chamber after the shot.
0,000000 0,000002 0,000004 0,000006 0,000008 0,000010 0,000012 0,0
0,2 0,4 0,6 0,8 1,0
Temperature, 2.5*106 K
Time, second
Figure 11. Temporal variation in vapour temperature in the reactor chamber after the shot.
After a time of 10 µs the pattern of vapour cooling changes. Condensation prevails over vaporization and the rate of temperature decrease slows down. Revaporization results in a density rise of one order of magnitude. The maximum mass of evaporated coolant is 14 kg.
The droplets of dispersed jets in the lower chamber section (see Fig. 4) provide the needed condensation surface (4.5 × 104 m2 for droplets with a total mass of 7,000 kg per shot). The change to a decreasing rate of condensation after 1 ms is due to the additional condensation of vapour on the dispersed jets after that time, which is needed for the uniform spreading of ionized vapour all over the chamber.
Vapour density practically reaches its saturation value at 0.01 s. This indicates that the condensation process would not limit a repetition rate of shots. Apparently, the actual limitation will be the rate of clean-up of liquid droplets. In the case of gravity-free precipitation the repetition rate does not exceed 2 Hz
6. NEUTRON HEATING OF BLANKET
The neutron source generated in a DT-reaction is presented in Figure 12, in accord with Ref. 5. The duration of the effective burning is equal to approximately 1 ns, so that we can consider the heating of the blanket instantaneous.
94 95 96 97 98 99 100
1E20 1E21 1E22 1E23 1E24 1E25 1E26 1E27 1E28 1E29 1E30 1E31
Neutron fluence, n/s
Time, ns
Figure 12. Neutron pulse of the FIHIF target micro-explosion. Solid line: 14 MeV; dotted line: 2.45 MeV.
The neutron spectrum is evaluated in stationary calculations using the MCNP code [12] in spherical approximation to the moment of the central peak of the neutron pulse. The average neutron energy is equal to 12.2 MeV. Two-dimensional calculations of neutron transport in the blanket by using the MCNP code arrived at a high energy release in the materials (see Table 2). The tritium-breeding ratio (TBR) for this blanket is equal to 1.112, and the blanket multiplication factor is evaluated at the value of 1.117. The neutron energy of 546 MJ is multiplied in the blanket up to a value of 610 MJ. Thus the total energy release per one shot is equal to 814 MJ.
The blanket is presented as a multi-layer cylinder the structure of which is given in Table 2.
The total blanket and the structural wall thickness is equal to 52 cm. We enumerated blanket zones and inner radii from the surface of the liquid protecting layer. The scheme of coolant motion through the vanadium tubes is presented in Figure 13.
18
Figure 13. Scheme of coolant motion through the vanadium tubes.
Table 2. Blanket structure and energy deposition in the zones Zone Matter Radius,
cm
Energy density, MJ/m3
Tempera- ture rise, K 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
PbLi SiC+PbLi PbLi V4Cr4Ti PbLi V4Cr4Ti PbLi V4Cr4Ti PbLi V4Cr4Ti PbLi V4Cr4Ti PbLi V4Cr4Ti PbLi V4Cr4Ti HT-9 Concrete
400.0 400.2 401.0 407.0 407.4 413.4 413.8 419.8 420.2 426.2 426.6 432.6 433.0 439.0 439.4 445.4 446.4 452.0
23.8 20.7 18.5 8.9 12.2 6.1 6.5 2.8 4.1 1.1 1.5 0.8 0.9 0.2 0.3 0.1 0.07 -
13.0 5.1 11.1 2.8 7.3 1.7 4.0 0.9 2.0 0.5 0.9 0.2 0.4 0.1 0.2 0.05 0.01 -
Neutron heating generates a pressure/stress pulse which travels across the blanket and refracts at the contact surfaces. For evaluation of material loading we solve 1D hydrodynamic equations in cylindrical geometry. The computational interval includes blanket, structural wall and concrete shield. Free adiabatic boundary conditions were used. Initial distributions were uniform for all the parameters except for internal energy which was determined from neutronics computations. The data for solid materials, silicon carbide, vanadium alloy and stainless steel are given by Zinkle in ref. [12]. The eutectic Li17Pb83 properties are taken from Ref. [13]. The equation of state is taken in the Mie-Gruneisen form. The Gruneisen coefficient Γ is projected as 2 for the SiC porous wall, stainless steel and concrete. In Figs 14 the pressure distribution in the blanket and the structural wall is drawn for different moments of time. The right boundary is the contact interface between stainless steel and concrete. The main pulsations are modulated by low frequency which can be caused by refracted pulse reflection
in concrete. Small-amplitude high-frequency pulsations are produced by the tubing walls reverberations.
4,0 4,1 4,2 4,3 4,4 4,5
-40 -20 0 20 40 60
300µs 200µs 100µs
time=0µs
Pressure, MPa
Radial distance, m
Figure 14. Pressure distribution in the FIHIF reactor chamber blanket and the structural wall at various times.
0 5 10 15 20
0,0 0,2 0,4 0,6 0,8 1,0
VCrTi SiC+PbLi
(3J2/2Y2)1/2
Time, ms
Figure 15. Temporal variation of equivalent stress normalized by yield stress Y (Y= 35 MPa for SiC/PbLi and 280 MPa for V-Cr-Ti). The von Mises criterion (σe≤Y) determines elastic/plastic material behaviour.
In the walls a control of stresses with respect to the strength material behaviour is needed. In Figure 15 the equivalent stress σe=(3J2/2)1/2, normalized by the yield stress Y, is plotted versus time for two construction walls: the first wall of SiC+PbLi and adjacent to it a V-Cr-Ti wall. The equivalent stress σe in the first wall is quite near to the yield stress Y, while in the vanadium alloy wall σe is substantially less than Y. According to the von Mises criterion
e≤Y
σ both materials are in the elastic mode of loading.
7. ENERGY CONVERSION AND BALANCE OF PLANT SYSTEM
The energy conversion system consists of three loops. The coolant of the second loop is sodium. The third loop is a steam turbine cycle. The key parameter of the system is the maximum temperature of Li17Pb83 at the outlet of the reactor chamber. It is taken as 823K.
The inlet temperature of the eutectic is equal to 623K. The inlet and outlet temperatures of sodium in the intermediate heat exchanger are 573K and 773K, respectively. In Table 3 the mass flow rate and the pump power for liquid metal coolants are given.
20
Table 3. Parameters of thermal loops of an FHIF power plant First loop
Coolant LiPb Mass flow rate, kg/s 13063
Pump power, kW 11584
Second loop
Coolant Na Mass flow rate, kg/s 6402
Pump power, kW 3768
Steam Cycle
Mass flow rate, kg/s 548.7 Inlet pressure, MPa 18 Superheat pressure, MPa 3 Condenser pressure, MPa 0.009 Turbine efficiency 0.875 Steam cycle efficiency 0.417 Reactor
Fusion power, MW 1500 Driver power, MW 60 Neutron power ratio 0.773 Blanket multiplication 1.117 Power plant
Thermal efficiency 0.407
Net efficiency 0.374
Net power output, MW 626
The steam cycle is configured with a reheat. The initial steam temperature and the reheat temperature are equal to 743 K. The temperature of feeding water is computed as 450 K. The efficiency of the steam cycle equals 0.407. Taking into account the driver efficiency, the target gain and the blanket multiplication for the fusion power of 1500 MW, we obtain the net efficiency of the plant of 0.373 and the net power per one reactor of 626 MW.
The thermal scheme of an FIHIF power plant is presented in Figure 16.
Figure16. Thermal scheme of an FIHIF power plant.
8. CONCLUSIONS
The FIHIF physical scenario based on the high-energy ion beams drive of a cylindrical target involves a realistic acceleration technology and a simple target design. The configuring of the reactor of two sections gives a possibility to increase the rate of vapour condensation and to reduce the vapour pressure loading. The problems of vapour fog in the chamber and pressure- stress pulsations in the blanket are a major concern of the reactor. The material limitations and tritium contamination of coolant determine the thermodynamics of the thermal scheme. The balance of the plant is highly influenced by the driver energy consumption. An improvement in material characteristics and the driver–target performance are needed to increase the efficiency of the power plant.
ACKNOWLEDGEMENTS
This work is sponsored by the Human Capital Foundation under contract N32. It is supported in part by the Programme of the Mathematical Department of the Russian Academy of Sciences N3.
REFERENCES
[1] J.D. LINDL et al., Plasma Phys. Contr. Fusion 45 (2003) A217.
[2] W.R. MEIER et al., Fusion Eng. Des. 62–63 (2003) 577.
[3] S.A. MEDIN et al., Laser Part. Beams 20 (2002) 419.
[4] S.A. MEDIN et al., Fusion Sci. Technol. 43 (2003) 437.
[5] M.M. BASKO et al., Laser Part. Beams 20 (2002) 411.
[6] R.W. MOIR, Fusion Eng. Des. 32–33 (1996) 93.
[7] HUBBELL J.H., SELTZER S.M. Tables of X-Ray Mass Attenuation Coefficients. NIST (1996).
[8] V.P. KOPYSHEV, A.B. MEDVEDEV. Thermodynamic model of compressing covolume. Sarov: VNIIEF (1995) (in Russian).
[9] B. ROSSI, High Energy Particles, Prentice-Hall, Englewood Cliffs, New Jersey (1952).
[10] V.P. ISACHENKO, Heat Transfer in Condensation Processes, Energia, Moscow (1977) (In Russian).
[11] GROUP-6, “MCNP – A General Monte Carlo Code for Neutron and Photon Transport”, LA-7396-m revised, Los Alamos National Laboratory (Apr. 1981).
[12] S.J. ZINKLE, Status of Recent Activities by the APEX Material Group, APEX Study Meeting, Sandia National Laboratories (1998) 18.
[13] V.N. MIKHAILOV et al., Lithium in fusion and space power in the 21st century, Nauka, Moscow (1999) (in Russian).
22
The Mercury Laser, a gas cooled 10 Hz diode pumped Yb:S-FAP system for inertial fusion energy
C. Bibeau, A.J. Bayramian, P. Armstrong, R.J. Beach, R. Campbell, C.A. Ebbers, B.L. Freitas, T. Ladran, J. Menapace, S.A. Payne, N. Peterson, K.I. Schaffers, C. Stolz, S. Telford, J.B. Tassano, E. Utterback
Lawrence Livermore National Laboratory, Livermore, California, United States of America Abstract. The Mercury laser project is part of a national inertial fusion energy programme in which four driver technologies are being considered, including solid-state lasers, krypton fluoride gas lasers, Z-Pinch and heavy ions. These drivers will be evaluated on several important criteria including: scalability, efficiency, reliability, cost, and beam quality. Mercury’s operational goals of 100 J, 10 Hz, 10% efficiency in a 5 times diffraction limited spot will demonstrate the critical technologies before scaling the system to the multi- kilojoule level. Operation of the Mercury laser with two amplifiers has yielded 30 Joules at 1 Hz and 12 Joules at 10 Hz with over 8 104 shots on the system. Static distortions in the Yb:S-FAP amplifiers were corrected by a magneto-rheological finishing technique.
1. INTRODUCTION
The Mercury laser was initially commissioned with a single amplifier module to test the basic architecture and assess the performance of several component technologies: high-power diode arrays, Yb:S-FAP crystals, gas cooling, and a high average power Pockels cell [1]. In this paper we report on the first integrated operation of the system with two amplifiers in an upgraded facility that includes Class 100 clean room systems, enhanced controls and several diagnostic stations (Figure 1). The primary focus has been on energy and average power operation at 1.047 um, but there will be added active wavefront control and average power frequency conversion capabilities to meet our requirements for a scalable inertial fusion driver.
The Mercury laser project is part of a national inertial fusion energy programme in which four driver technologies are being considered, including solid-state lasers, krypton fluoride gas lasers, Z-Pinch and heavy ions. These drivers will be evaluated on several important criteria including: scalability, efficiency, reliability, cost, and beam quality. Mercury’s operational goals of 100 J, 10 Hz, 10% efficiency in a 5 times diffraction limited spot will demonstrate the critical technologies before scaling the system to the multi-kilojoule level.
2. CRYSTALS AND GAS COOLING
In the process of down-selecting the optimum gain medium, we explored numerous gain materials and architectural options to best fit the design space of a fusion energy class laser.
Yb3+:Sr5(PO4)3F or Yb:S-FAP was chosen as the gain medium, based on its long energy storage lifetime, suitable absorption and emission cross sections, and good thermal conductivity. Although the fabrication of full aperture Yb:S-FAP slabs of 4 × 6 × 0.75 cm3 has been quite challenging, we have made good progress towards growing large enough boules to produce full-size slabs. Successful growth techniques by Northrop Grumman Inc.
have yielded 6.5 cm diameter by 10 cm long boules from which 2 full-size amplifier slabs can be fabricated. The next challenge has been cutting the boules without fracturing, since the growth techniques employed to eliminate or reduce crystalline defects have produced boules
that are difficult to cut due to the high residual stress from the growth process. We have found that water jet cutting of the material is gentle compared to other cutting techniques and is now the standard process for cutting Yb:S-FAP.
a)
Figure 1. A picture of the Mercury laboratory showing dual amplifiers and Class 1000 clean room enclosures which minimize air turbulence and decrease optical damage due to contaminants.
After cutting the boule, the crudely shaped slabs are polished into their final form. If the transmitted wavefront does not meet the λ/10 peak-to-valley or λ/90 cm–1 gradient specification, the slab is re-polished by magneto-rheological finishing (MRF), which is a method of deterministically removing material from the surface. The MRF technique is capable of removing features down to the 1 mm scale and to several waves in amplitude. An example of a slab polished by the MRF technique is shown in Fig. 2, where the stress-induced distortion is removed from the bonding process. In addition to the wavefront distortion, we are also concerned with the optical lifetime of the slabs. Employing established polishing technology adapted to the Yb:S-FAP material, subsurface damage is removed and the surface micro-roughness can be reduced to less than 3nm. The measured damage threshold at 1047 nm of conventionally or MRF polished Yb:S-FAP substrates has increased to 18 J/cm2 at 3 ns. The slabs are mounted with a compliant urethane compound into aerodynamic aluminum structures called “vanes”. The faces of the two crystals are separated by 1 mm in helium gas cooling channels where the typical gas velocity exceeds Mach 0.1 at four atmospheres of helium pressure.
3. LASER DIODES AND ARCHITECTURE
The laser diodes emit at 900 nm to overlap the 4.6 nm wide Yb:S-FAP absorption line. The diodes are precision-mounted onto etched silicon heatsinks with microlenses to increase brightness by 10×, then secured onto large copper cooling blocks [2]. Each amplifier assembly is pumped by four 80 kW diode arrays. The diode arrays are separated to allow for the passage of the 1047 nm extraction beam through the center. The diode array light is guided to the amplifier through multiple reflections within a hollow, highly reflective metal lens duct and a parallel plate homogenizing structure.
24
a)
PV: 1.338 um Rms: 0.130 um
2-D 3-D
b)
PV: 0.0922 um (14.5x) Rms: 0.0191 um (7x)
2-D 3-D
Figure 2. a) Initial phase of bonded slab showing 1.338 micron wavefront distortion, and b) Final phase of amplifier slab after MRF treatment showing 0.0922 micron wavefront distortion.
The overall architecture employs a 5 mrad angularly multiplexed, four-pass beam layout.
Image relaying of the laser beam with telescopes between the amplifiers helps to greatly reduce the intensity modulation at the crystalline amplifiers. An average-power, birefringence-compensated Pockels cell [3] is inserted after two passes at low fluence (<1 J/cm2) to help suppress the energy buildup of any parasitic beams.
The laser system, diode laser power conditioning, and utilities are all computer-controlled. A full suite of sensor packages has been fielded to diagnose the beam after each pass. The most important diagnostic is the dark field which allows rapid detection of 100 um size damage in the amplifier relay plane at 10 Hz. When the computer software algorithm detects a change in the dark field image, a signal is sent to the control system to shut down the laser.
4. LASER EXPERIMENTS
During the first commissioning of the amplifier seven Yb:S-FAP slabs were utilized. The multipass architecture was still incorporated to test the 4-pass architecture concept [1]. These tests were performed before MRF had been utilized on bonded Yb:S-FAP slabs. To compensate for the distortions, a static phase plate was made with MRF technology to correct collective distortions of all the slabs. We measured a factor-of-three improvement in the energy within a diffraction-limited spot (Figure 3a). The energetics data curve for the single amplifier system was mapped by keeping the front-end energy constant and increasing the diode pump pulsewidth to increase the gain. The data shows reasonable overlap with our energetics model with no adjustable parameters (Figure 3b). Utilizing 360 mJ of front energy and pumping for up to 1 millisecond, we were able to extract up to 33.4 joules of energy at 1047 nm, which corresponds to a 4.6% electrical to optical efficiency. Increasing the pump repetition rate to 5 Hz, 114 W of average power was achieved with <0.5% rms energy fluctuation (Figure 3c) for 20 minutes. The single amplifier activation campaign had an accumulation of greater than 3.8 × 104 shots. The beam quality or M2 of the output beam was captured at this highest power level and was found to be 2.8 × 6.3 times diffraction limited, with the larger divergence associated with the bond distortion.
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